A concrete tunnel lining can be formed using a pumping device for feeding extruded concrete into a tunnel and a form including an interior form member and a forwardly movable form front having a plurality of concrete input ports distributed uniformly around the annular space. The extruded concrete is supplied at once to an entire annular axially extending section. A new inner lining can be inserted and the process repeated to line the tunnel.
As described in German Patent DE-PS No. 34 06 980 concrete is pumped behind a tunnel excavator through a single upper opening in a forwardly sliding form front.
In order to reliably support the surrounding ground behind the tunnel excavator in loose earth, a steady pressure of flowing concrete behind the form front must be guaranteed which is higher than the ground pressure operating on the tunnel liner and the pressure or load due to the ground water. In order to guarantee that this pressure is continuous, a variety of precautions must be taken for the proper support of the form front and feeding of the concrete.
Although by a resilient support of and a controlled forward motion of the form front individual prerequisites for a satisfactory extrusion process are satisfied, a completely unobjectionable concrete extrusion process has yet to be attained.
In practice the concrete pumped through the form front is not deposited layerwise parallel to the forwardly sliding form front as one might expect, but flows in nonpredetermined channels inside of the previously pumped in concrete. Thus a clumplike structure results in the continuously forming tunnel liner. At the edge of this clumplike structure the concrete is not thick. It forms a more heterogenous separate zone of so-called nests and holes.
Even disregarding that the strength of this concrete is below normal and even requisite standards, the inferior quality of the concrete is a source of danger in water bearing loose soil. Water mixed with earth can be forced through holes in the tunnel liner into the tunnel interior. It can endanger the tunnel bed and the stability of the tunnel.
Our research has shown that the flow of concrete into the circular space, which is bounded on the inside by the steel interior form member, on the outside by the surrounding ground, and to the front by the forwardly sliding form front, is a process which is subjected inter alia to the following several factors:
The fluidity of the concrete depends particularly on its specific material properties and on the time, since a chemical reaction is involved.
The surrounding earth influences the fluidity by the roughness of the surface interacting with the concrete and by providing a site through which water lost from the concrete is filtered. As water is lost from the concrete the fluidity is significantly decreased.
The hydrostatic pressure which is generated in the circular space and together with it the position of the concrete input ports in the form front.
The laws of fluid mechanics apply since through the pump opening or concrete input ports in the form front positioned adjacent the upper circular space concrete flows at a reduced pressure into the top of the form in the still softened state which has a reduced shear strength. After extending several meters into the region in which the setting process is in progress and thus the shear strength is increasing, the downward concrete flow is diverted into a region with larger hydrostatic pressure. Through the lower concrete input ports pumped concrete flows through a pressure gradient immediate behind the form front.
The clumps, which are parts or regions of the extruded concrete, act as cohering surfaces behind the forwardly sliding form front and harden there. As a result in this region the fluidity of the concrete is a little less or the pressure potential lines remain constant for a longer time i.e. the pressure gradient may be zero.
The clumps are observed particularly in the region between two concrete input ports in the form front, when the spacing between the concrete input ports is very large. The clumps are therefore a problem, because they circulate under high pressure at the end of the concrete facing the form front and therefore follow the form front. Some of the flow channels behind the clumps collapse or the friction on the ground increases. Then the clumps gather together. At that moment a reduced pressure or a gap arises in the end of the concrete closest to the form front, when the concrete cannot immediate fill up the space between the form front and the remaining clumps as it slides forward.